In a powder metallurgy process for manufacturing a product such as a sintered body using a principal material powder such as an iron-based powder, the principal material powder is mixed with an auxiliary material powder such as a graphite powder in order to improve physical properties, including a strength property and a processing property, of the sintered body, and the mixed powder is formed into a green compact by pressing. The green compact is sintered at a temperature equal to or lower than the melting point of the principal material powder to produce the sintered body.
It is generally pointed out that problems exist in a method of manufacturing such a sintered body. For example, it is known that when the formed green compact is extracted from a die, a friction coefficient between the green compact and a die surface may be increased, which causes die galling, i.e., scratches on the surface of the die, or damage to the die. A lubricant (for example, as in PTL 1) or the like therefore may be mixedly added into a mixed powder including a principal material powder and an auxiliary material powder in order to improve lubricating ability.
For example, the graphite, which is generally used as the auxiliary material powder, has a lower specific gravity and a smaller grain size than the iron-based powder as the principal material powder. It is therefore generally pointed out that if the graphite is simply mixed with the iron-based powder, they are greatly separated from each other and segregation of graphite occurs, which disadvantageously prevents uniform mixing. If the carbon content in a green compact varies due to the graphite segregation, carbon concentration in a sintered body also varies, and thus the sintered body has unstable mechanical properties. Consequently, it is difficult to produce stable components.
Furthermore, use of graphite causes scattering of the graphite and in turn causes dust emission in a mixing or pressing step. This disadvantageously lowers the graphite content in the mixed powder, and degrades working environment due to the graphite dust emission. Such segregation or dust emission occurs not only for the graphite but also for any of other powders to be mixed with the principal material powder. Hence, it is required to prevent such segregation or dust emission.
Adding a binder to the mixed powder, the binder acting as a bond, is therefore suggested as a technique for preventing the segregation or dust emission. For example, a rubber binder such as styrene-butadiene rubber (SBR) is provided as the binder (for example, PTL 2).
However, the rubber binder is highly adhesive, and thus causes the mixed powder to be less fluid. If the mixed powder is less fluid, the following problems occur. For example, in a pressing step, when the mixed powder is discharged from a storage hopper and transferred into a forming die, or when the mixed powder is filled in the forming die, the mixed powder is not smoothly discharged due to bridging or the like above a discharge port of the storage hopper. Moreover, a hose from the storage hopper to a shoe box is blocked. Furthermore, if the mixed powder is less fluid, the mixed powder is less likely to be filled in the entire inside, particularly a narrow space portion, of the forming die, making it difficult to produce a homogenous green compact.
To solve such problems, PTLs 3 to 10 each disclose a resin binder that prevents the segregation or dust emission and furthermore improves fluidity of the mixed powder, and an auxiliary additive for the resin binder.